1. Field of the Invention
The present invention relates to the field of integrated circuit testing devices. The present invention more specifically relates to an assembly ring for a contact transfer interface between a test head of a testing device and a conductive tip board itself transferring the contacts from the interface to an integrated circuit chip to be tested.
2. Discussion of the Related Art
FIG. 1 very schematically shows a conventional integrated circuit testing system.
A testing device generally includes a test head 1 connected, by wire links (not shown), to a programmable control system (not shown) that can generate test signals and process, in return, informations received from the test head.
Head 1 includes, in a free surface, directed to an integrated circuit 2 to be tested, contact making protruding rods 3. Rods 3 are meant to ensure an electric contact with a printed circuit wafer 4 forming an electric interface between rods 3 of head 1 and a tip board 5. Tips 5' and rods 5" are meant to transfer contacts from interface 4 to an integrated circuit chip 2.
The function of tip board 5 is to connect contacts (not shown) of an integrated circuit chip 2 with a given spacing, depending on the type of chip involved, to contacts (not shown) of a front surface of printed circuit wafer 4 which are not necessarily at the same spacing as the contacts of chip 2. Board 5 then generally includes deformable or supple tips 5' having great length.
The function of printed circuit wafer 4 is to transfer its front surface contacts to contacts of its rear surface which are located facing rods 3 of the test head.
The pattern of the rear surface contacts of wafer 4 thus depends on the test head while the pattern of the front surface contacts depends on the type of chip to be tested.
Generally, rods 3 of test head 1 and/or rods 5" of board 5 are telescopic and fitted with springs.
It should be noted that designations "front surface" and "rear surface" of wafer 4 are used here by mere convention and are not meant to be limiting. Wafer 4 is thus provided, on each of its surfaces, with conductive tracks and includes vias of contact transfer from one surface to the other.
The testing of integrated circuit chips is generally performed in batches on semiconductive wafers 6, for example made of silicon. The wafers are successively brought on a test plate 7 and the chips 2 of each wafer 6 are successively brought above tips 5' of board 5 to perform the testing of the concerned chip.
Generally, all test procedures, that is, both the generation and the processing of the electric signals, and the displacement of wafers 6 as well as the passing from a wafer to the next one in the same batch, are automated.
Rods 3 of the test head are generally distributed by blocks 8 at the periphery of the lower surface of the test head to face peripheral areas of wafer 4. Peripheral blocks 8 are generally used to carry low frequency signals (in particular, the supply signals), and additional rods (not shown) for carrying high frequency signals are generally provided in the central portion of the test head.
Since a test head is meant to test different types of chips, interface wafer 4 and tip board 5 are removable, and one wafer 4 and one tip board 5 are available for each type of chip 2.
A problem which arises in this type of testing device is associated with removably maintaining a given interface wafer 4 in position of association with the test head.
A ring 9 for tightening a wafer 4 against head 1 is generally used. Such a ring is generally formed by a generally cylinder-shaped collar, a lower edge 10 of which, directed towards its center, forms a support for the periphery of a wafer 4 positioned inside the collar. Opposite to edge 10, collar 9 comprises punctual protrusions 11 directed towards the inside. Protrusions 11 are meant to cooperate with cams 12 protruding from the external periphery of test head 1 to cause, by a rotating motion of ring 9, a blocking of wafer 4.
For the wafer to maintain a correct position with respect to rods 3 despite the rotating motion of ring 9, wafer 4 generally includes bores (not shown) for cooperating with the positioning rods (not shown) protruding from the lower surface of test head 1.
The distance separating test head 1 from wafer 4 is generally adjusted by means of shims 13 interposed between the rear surface of wafer 4 and the lower surface of head 1.
A problem which arises, upon assembly of a printed circuit wafer 4 on a test head 1, is that the tightening performed by means of ring 9 tends to cause a bending of wafer 4, as illustrated in FIG. 1. Such a deformation of wafer 4 risks breaking the conductive tracks of this wafer and risks bad contact with rods 3 of the test blocks, especially for high frequency contacts performed in the central portion of the wafer.
Further, the flexing of interface wafer 4 is transmitted to the connection between the front surface of wafer 4 and tip board 5.
FIGS. 2A and 2B partially show, respectively, in cross-sectional view associated with a wafer 4 and in top view, an assembly ring 9' according to a conventional solution to solve this problem of flexing of the interface wafer.
This solution consists of enlarging edge 10 supporting wafer 4 so that this edge extends to the center of the wafer, beyond above rods 3 of peripheral blocks 8 of test head 1. The assembly ring being generally a metal ring, isolating shims 14 are then provided on the internal surface of edge 10 to prevent an electric contact with tracks of the front surface of wafer 4.
A drawback of this solution is that the friction caused by the rotation of ring 9' upon assembly of a wafer 4 causes a quick wearing out of the interface wafer and damages its front surface conductive portions.
Another disadvantage of conventional tightening rings such as shown in FIGS. 1 and 2 is that the assembly of the ring on test head 1 requires the use of a tightening tool due to the friction between edge 10 and wafer 4 (FIG. 1) or to the friction between shims 14 and wafer 4 (FIG. 2).